1. Field of the Invention
This invention relates to photoresist developer compositions and methods of use, and more particularly to metal ion-based, aqueous developer compositions for developing electron beam exposed positive photoresists and method therefore.
2. Description of Related Art
Shrinking semiconductor device feature sizes and increasing circuit densities necessitates improvements in semiconductor fabrication processes and materials, in particular submicron lithography tooling and high-resolution resist compositions. Device fabrication requirements such as high-resolution, and tight overlay creates a need for resists and resist/developer combinations exhibiting high dry etch resistance, high-resolution, high speed, and adequate process and line width control.
The smallest feature (i.e., opening or space) that can be produced in a photoresist layer is referred to as its resolution capability. The smaller the feature produced, the better the resolution capability. Presently, features on an integrated circuit require wafer resist resolution capabilities of about 0.25 microns (.mu.m). The effort to pack ever-increasing functional density on a semiconductor die, however, results in smaller, more densely packed, device elements. Speed and power consumption requirements of these high density integrated circuits further drive the device designer to use increasingly smaller dimensions. It is anticipated that the smallest feature size in an integrated circuit device will approach 0.13 .mu.m within the next five years.
Fabrication of these semiconductor devices using photolithographic processes includes forming an image on a semiconductor wafer using a mask. The mask includes a transparent substrate, generally quartz, and a thin layer of patterned material, typically 800 .ANG. of chrome. A photoresist is applied over the mask surface to a thickness of between about 2000 .ANG.-6000 .ANG.. The patterned image on the mask is typically four to five times larger than the circuit to be imaged onto the semiconductor wafer. The reduced image is formed on the semiconductor wafer photoresist by passing actinic radiation through the mask, and focusing the reduced image on the wafer photoresist.
The fabrication of a 0.13 micron (.mu.m), or 130 nanometer (nm), device on the wafer necessitates improvements in mask making processes and materials. Ideally, a 4.times. mask must have a resolution of about 0.52 .mu.m in order to provide 130 nm feature resolution on the wafer. However, proximity effects caused by diffusion of radiation in the wafer photoresist reduces the minimum feature size required on the mask to approximately 0.26 .mu.m. In some cases, even smaller feature sizes may be required in order to correct optical proximity effects. The Semiconductor Industry Association, a trade association of semiconductor device manufacturers, indicates a minimum feature size of 0.20 .mu.m on the mask in order to accommodate a 130 nm wafer minimum feature size.
The minimum feature size formed in a photoresist is determined by, among other things, the wavelength of the exposing energy. Resolutions on the order of 130 nm on a wafer require short wavelength radiation such as extreme ultraviolet (where the wavelength, .lambda., is approximately 13 nm), x-rays (.lambda. of approximately 0.1-5 nm), and high energy electron beams. Accordingly, masks having less than a 0.20 .mu.m minimum feature size include fabrication processes which must be restricted to these energetic, short wavelength radiation sources.
As electron beam energies provide sufficiently short wavelengths, high resolution masks are typically patterned using a computer-controlled, electron beam writing tool. The electron beam exposes preselected portions of an electron-beam sensitive photoresist deposited on a glass or quartz plate overlaid with a thin layer of metal or metal oxide. The electron beam resist is developed and the exposed metal or metal oxide is etched in the pattern of the desired circuit to produce a mask (for full-wafer exposure), or a reticle containing the pattern for a few semiconductor dies, or one die.
Presently available electron beam resists include poly(methylmethacrylate) ("PMMA"), poly(methyl-isopropenylketone) ("PMIPK"), poly(butene-1-sulfone) ("PBS"), poly(trifluoroethyl chloroacrylate) ("TFECA"), poly(.alpha.-cyanoethylacrylate-.alpha.-amidoethylacrylate) copolymer ("PCA", and poly-(2-methylpentene-1-sulfone) ("PMPS"). These resists are inconvenient in that the PMMA is very insensitive to electron beam and actinic radiation, and the PBS and PMPS degrade at higher temperatures. Such thermal degradation results in low dry etch resistance, thereby requiring that the mask's underlying chrome layer be wet etched. One problem with wet etching, however, is that the etchant undercuts the exposed metal film, widening the feature size by approximately 0.12 .mu.m on each side. In the case of a space, or channel, in the mask's metal film, the channel width is increased by 0.12 .mu.m on both sides thereby increasing the channel width by 0.24 .mu.m over the original, as-exposed feature size. The resulting feature is larger than the desired minimum feature size of 0.20 .mu.m required to obtain 130 nm resolutions on the wafer. Accordingly, these sulfone-based electron beam resists are unsuitable for masks where a resolution capability of less than 0.20 .mu.m is required.
Another problem with the above listed electron beam resist compositions is that they require an organic solvent, typically methylisobutyl ketone (MIBK), methylisoamyl ketone (MIAK), methylpropyl ketone (MPK), ethoxyethyl acetate, or 2-methoxyethylene. Moreover, these resists require that the developer solutions also be based on an organic solvent. These solvents represent significant health hazards, and/or flammability hazards, therefore requiring special handling, and disposal considerations. The federal and local regulations governing the use, and disposal of these solvents include abatement equipment, special storage facilities, and monitoring requirements significantly increase the manufacturing costs associated with using an electron beam resist.
Yet another problem with electron beam resists is their relatively low contrast. The pattern resolution attainable with a given resist for a given set of processing conditions is determined, in part, by the resist contrast (.gamma.). Referring to Prior Art FIG. 1, for a positive resist, the film thickness of the irradiated region decreases gradually with increasing radiation exposure, until eventually the clearing dose D.sub.c is reached, resulting in complete removal of the film upon development. Accordingly, D.sub.c defines the "sensitivity" of a positive resist.
Contrast (.gamma..sub.p) is related to the rate of degradation of molecular weight of the exposed resist and is defined as: EQU .gamma..sub.p =1/[log.sub.10 D.sub.c -log.sub.10 D.sub.0 ] Equation (1)
where D.sub.o is the dose at which the developer begins to attack the irradiated film and is defined as the intersection of the extrapolated linear portions of the normalized remaining film thickness versus dose plot. A higher contrast value renders non-exposed portions of the resist less susceptible to photodissolution resulting from scattered reflected radiation by a developer. As a result, higher resolutions are characterized by features having crisp, clean edges thereby starkly delimiting the exposed resist regions from unexposed resist regions.
Still referring to FIG. 1, the parameters defining the resist characteristics include the "dark loss." Dark loss represents the thickness of unexposed resist that is removed by the developer. When dark loss is large, thicker resist films must be initially applied so that the resulting thinner, developed film is able to adequately protect the underlying metal film area of the mask during dry etch. The capability of a particular resist relative to resolution and thickness is measured by its "aspect ratio." The aspect ratio is calculated as the ratio of the as-applied resist thickness to the minimum attainable width of the resist opening after developing.
One problem associated with applying a thicker resist film to compensate for dark loss is that the aspect ratio limitations result in lower resolution capability, longer development times, and larger minimum feature sizes. Accordingly, there is a need for a resist developer composition and method exhibiting lower dark loss to permit using thinner resist films thereby enhancing the resolution capability of the resist.
Positive photoresists such as diazonapthaquinone (DNQ) sensitized phenolic resin (known as novolak resin) are widely available for wafer photolithography processes for producing semiconductor devices. However, these positive resists are optimized for maximum sensitivity to the ultraviolet portion of the electromagnetic spectrum (particularly I-, G-, and H-lines of a mercury vapor lamp) and their performance is linked to proprietary developer formulations that maximize the contrast and speed of the resist at these exposure wavelengths. These developers are typically an aqueous organic-alkali developer such as quaternary ammonium hydroxide. These DNQ/novolak positive resist/developer systems, however, exhibit poor photosolubilization when exposed to an electron beam.
Attempts to overcome the limitations of electron beam resists have failed to attain an on-mask minimum feature size of less than 0.20 .mu.m. U.S. Pat. No. 5,223,377 to Samarakone et al. discloses an interrupted developing process for an on-wafer photoresist. The on-wafer photoresist is exposed to low energy radiation through a mask to thereby activate the photoresist. The mask is removed and the exposed on-wafer resist is subjected to a developer. Samarakone et al. does not disclose first attaining a minimum feature size of less than 0.20 .mu.m on a mask. Accordingly, Samarakone et al. is not helpful in identifying processes and compositions that will enable attaining an on wafer minimum feature size of 130 nm.
Lazarus et al., U.S. Pat. No. 5,094,934 discloses an organic alkali developer, including quaternary ammonium hydroxide, and an alkanolamine or morpholine adjunct, for developing on-wafer quinone diazide sensitized positive photoresists. As in Samarakone, Lazarus discloses an on-wafer resist, and exposure of the resist film to mercury vapor lamp generated ultraviolet radiation through an exposure mask. Lazarus does not teach how to achieve an on-mask minimum feature size of less than 0.20 .mu.m. Moreover, their use of ultraviolet wavelengths to expose the on-wafer resist indicates on-wafer minimum feature sizes in excess of 300 nm.
Improvements in the resist development process have also failed to provide an on-mask minimum feature size of less than 0.20 .mu.m. Chiong al., "Contrast and Sensitivity Enhancement Of Resists For High-Resolution Lithography", J. Vac. Sci. Technol. B, Vol. 6, p. 2238, November/December 1988, report improvements in contrast profiles of novolak-based photoresists exposed to an electron beam when an interrupted development cycle is carried out. Moreau teaches a repetitive development cycle including a long immersion time in a potassium hydroxide (KOH) developer, followed by a brief deionized water rinse, blow dry, and re-immersion. Moreau reports minimum feature sizes of approximately 0.25 .mu.m. However, these features exhibit significant amounts of scum (i.e., incomplete development). Over-development of the resist to reduce the scum resulted in significant undercutting of the novolak resist.
Accordingly, commercially available electron beam resists exhibit low contrast, are incapable of providing the less than 0.20 .mu.m minimum on-mask feature size necessary to enable photolithographic production of 130 nm semiconductor devices, and/or their organic solvent systems, and organic solvent-based developers render then hazardous and undesirable in a manufacturing environment. Also, positive photoresist systems such as phenolic-based novolak resins in combination with organic alkali developers exhibit high dark loss, poor contrast and poor speed when an electron beam is used to expose the resist. Process enhancements in developing phenolic-based resin resists, while improving the resolution capability of the resist fall short of providing a developed resist having minimum feature sizes less than 0.20 .mu.m. There is a need for a resist/developer/process combination capable of providing resolution capabilities with high contrast and low dark loss.